This invention relates in general to anti-lock brake systems and in particular to a spin-and-brake algorithm which includes a test to verify that the vehicle is actually in a spin-and-brake situation.
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. One type of ABS controls only the vehicle rear wheel brakes. Such a system is referred to as a Rear Wheel Anti-Lock Brake System (RWAL) the following.
A typical prior art RWAL is illustrated generally at 10 in FIG. 1. As shown in FIG. 1, the RWAL 10 is installed on a vehicle having a hydraulic braking system consisting of a brake pedal 12 coupled to operate a dual reservoir master cylinder 14. When the vehicle operator depresses the brake pedal 12, the master cylinder 14 supplies hydraulic fluid under pressure from a front reservoir 14a through a hydraulic line 16a and from a rear reservoir 14b through a hydraulic line 16b to a conventional combination or proportioning valve 18. The combination valve 18 includes a first output line 18a adapted to supply hydraulic fluid at a first predetermined pressure to actuate a pair of vehicle front wheel brakes 19a and 9b. The combination valve 18 also includes a second output line 18b which supplies hydraulic fluid at a second predetermined pressure to actuate a pair of vehicle rear wheel brakes 20a and 20b.
The RWAL 10 shown in FIG. 1 utilizes a control valve 21 to selectively control the application of pressure to the rear wheel brakes 20a and 20b when the system is in an anti-lock braking mode. The control valve 21 includes a normally open solenoid valve 22 connected between the line 18b and a line 24 which supplies pressurized brake fluid to the controlled rear wheel brakes 20a and 20b. During an anti-lock braking cycle, the normally open valve 22 isolates the rear wheel brakes 20a and 20b from the master cylinder 14 and is commonly referred to as an isolation valve. The isolation valve 22 also can be selectively opened to increase the pressure at the rear wheel brakes 20a and 20b.
The control valve 21 also includes a normally closed solenoid valve 26, which is connected between the line 24 and a fluid accumulator 28. The normally closed valve 26 is commonly referred to as a dump valve. The dump valve 26 is selectively opened to reduce the pressure at the rear wheel brakes 20a and 20b by bleeding brake fluid from the rear wheel brakes to the accumulator 28. In the RWAL 10, the master cylinder 14 provides a source of pressurized hydraulic brake fluid during an anti-lock braking cycle, thus eliminating the need for a separate source of pressurized hydraulic fluid, such as a motor driven pump, which is usually included in a four wheel ABS.
The RWAL 10 further includes a computer control module 30 which is electrically connected to a wheel speed sensor 40. The control module 30 can be mounted directly upon the control valve 21 or located remotely therefrom. The control module 30 includes a RWAL microprocessor (not shown) which is programmed to control the RWAL 10 in accordance with a RWAL control algorithm and parameters permanently stored in a Read Only Memory (ROM). The RWAL microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL illustrated in FIG. 1 is included in U.S. Pat. Nos. 4,790,607 and 4,886,322.
During vehicle operation, the microprocessor in the RWAL control module 30 continuously receives speed signals from the wheel speed sensor 40. The RWAL microprocessor monitors the speed signals for potential rear wheel lock-up conditions. When the vehicle brakes are applied and the RWAL microprocessor senses a first rear wheel speed departure, which is indicative of an impending wheel lock-up condition, the RWAL microprocessor is responsive thereto to close the isolation valve 22 to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. The RWAL microprocessor then selectively opens the dump valve 26 to reduce the pressure applied to the rear wheel brakes 20a and 20b and thereby correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve 22 to initiate a second wheel speed departure.
The operation of the RWAL 10 is illustrated by the graphs shown in FIG. 2. The upper solid curve labeled 60 represents the velocity of the rear wheels while the dashed curve labeled 61 represents the vehicle velocity. The operation of the isolation valve 22 and the dump valve 26 is illustrated by the curves labeled 62 and 63, respectively. The lower curve, which is labeled 64, shows the pressure applied to the controlled rear wheel brakes. During an anti-lock braking cycle, the first and second wheel speed departures are labeled 60a and 60b, respectively. Following correction of the second wheel speed departure, which occurs at time t.sub.7, the rear wheel brake pressure is maintained at a constant level P.sub.e.
With a RWAL system, it is necessary to identify and respond to a situation in which the vehicle operator has used the accelerator to spin the vehicle rear wheels faster than the actual vehicle speed and then steps on the brake pedal. Accordingly, if the brake pedal is depressed and the anti-lock braking mode entered while a rear wheel overspin condition exists, a "spin-and-brake" mode of RWAL operation is entered and a rear wheel overspin condition exists, as shown by the upper curve in FIG. 3. FIG. 3 also illustrates the operation of the isolation and dump valves 22 and 26 and the resulting pressure applied to the controlled wheel brakes vs. time.
In the spin-and-brake mode, the RWAL system control algorithm provides two cycles of pressure dumping followed by unlimited reapply until wheel speed departures are forced, as illustrated in FIG. 3. In FIG. 3, the upper curve 80 represents the actual rear wheel speed while the dashed curve 84 represents the actual vehicle speed. During the portion labeled 80a, the rear wheel speed is accelerated above the vehicle speed.
The RWAL control algorithm detects conditions which indicate that the vehicle has entered a spin-and-brake mode of operation. For example, if the rear wheel speed is 0.5 g greater than the projected vehicle speed ramp for more than 0.5 seconds, the ABS control algorithm sets a Spin-and-Brake (SPNBRK) flag TRUE. The SPNBRK flag is held TRUE for a relatively long predetermined time period, such as, for example 16 seconds. The long time out period for the SPNBRK flag compensates for the possibility that the rear wheel overspin continues at a nearly constant velocity for several seconds. Once the predetermined time period has elapsed, the SPNBRK flag is cleared. When the SPNBRK flag is TRUE, the control algorithm will enter the spin-and-brake mode upon application of the wheel brakes and entry into the anti-lock brake mode.
In FIG. 3, the SPNBRK flag is set TRUE during the portion of the curve labeled 80a. When vehicle brakes are applied at time t.sub.a, the RWAL control algorithm projects a speed ramp 86 and the wheel speed then decreases from the original overspin to actual vehicle speed while brake pressure is kept very low. In most instances, the pressure applied to the controlled rear wheel brakes, B.sub.1, is very close to zero. However, in order to illustrate the subsequent pressure reductions, B.sub.1 is shown above zero pressure in FIG. 3.
At tc, a first set of dump pulses 83a are generated to deal with a false wheel speed departure which occurs as the rear wheel speed decreases from the original overspin condition to the actual wheel speed. The RWAL control algorithm will then, at time t.sub.d, enter a "spin-and-brake" reapply mode and generate a first string of reapply pulses, which are labeled 82a, to cause a first real wheel speed departure cycle 80b. Also, at this time, the projected speed ramp 86 is reinitialized as a new projected speed ramp 87 at the present vehicle speed. After generating the spin-and-brake reapply pulses 82a to increase the pressure applied to the controlled rear wheel brakes 20a and 20b to a level B.sub.3, a series of dump pulses 83b are produced to reduce the pressure to B.sub.4 and correct the slip condition in the first real wheel speed departure cycle 80b. Next, one or more automatic reapply pulses 82b are generated to cause a second real wheel speed departure cycle 80c and increase the pressure to a level B.sub.5. Finally, a dump pulse 83b is generated to correct the slip condition in the second real wheel speed departure 80c and reduce the pressure to a level B.sub.6 for the remainder of the vehicle stop. A detailed description of the spin-and-brake mode of operation is included in the above-identified U.S. Pat. No. 4,790,607.